Prophylactic oophorectomy was performed in 28 female members of 16 families at high risk of ovarian carcinoma. 3 of these women subsequently developed disseminated intra-abdominal malignancy; there was uncertainty about the primary site despite extensive investigation. These tumours were indistinguishable histopathologically from ovarian carcinoma. It would seem that in cancer-prone families the susceptible tissue is not limited to the ovary, but includes other derivatives of the coelomic epithelium, from which primary peritoneal neoplasms may arise.